The prior art includes various methods for detecting the presence of a given frequency or band of frequencies in a composite signal. Pass filters typically used for this purpose include both passive RLC circuits and active RC circuits using operational amplifiers such as is shown in Eide U.S. Pat. No. 3,564,441. These filters, by themselves, usually have poor cut-off characteristics even when cascaded with each other. Other filter systems are made by inserting phase shifter circuits and then summing and differencing the output signals as suggested in Seidel U.S. Pat. No. 3,976,958 in order to achieve better sensitivity under some circumstances. Another circuit using phase shifting and summing to provide sharper detection is shown in Theobald U.S. Pat. No. 3,721,913. U.S. Pat. No. 3,381,220 to Burr shows a detection circuit using phase shifting plus digital detection techniques. Digital filter circuits which are useful in the present detector are shown in U.S. Pat. No. 3,912,917 to Nussbaumer, and in U.S. Pat. No. 3,988,606 to Eggermont.